Engines may operate using a plurality of different fuels, which may be separately delivered, or delivered in varying ratios, depending on operating conditions. For example, an engine may use a first fuel (e.g., ethanol) and a second fuel (e.g., gasoline), each with different knock suppression abilities, to reduce engine knock limitations while improving overall fuel economy. As another example, different fuels may result in different engine pumping work (for example, when the different fuels include a gaseous fuel versus a liquid fuel, or a port injected fuel versus a direct injected fuel). As still another example, different fuels may result in different parasitic losses (such as when the fuels include a fuel delivered via high pressure direct injection versus a fuel delivered via low pressure port injection). As still another example, an engine may use fuel injection and water injection.
Various approaches may be used to store different fuels and other substances (such as water for water injection) on-board a vehicle. For example, the different substances may be stored separately in different storage tanks, and thus filled separately. Alternatively, different substances may be stored in a mixed state (such as ethanol and gasoline), and then separated on-board the vehicle to enable individual control of delivery to the engine. For vehicle systems where the different substances are separated on-board, various separator systems may be included in the vehicle.
One example approach for on-board fuel separation is shown by Leone et al. in U.S. Pat. No. 7,845,315. Another example approach is shown by Dearth et al. in U.S. Pat. No. 8,015,951. The example approaches pressurize the fuel via an electric pump and then heat the pressurized fuel upon passage through a heat exchanger, the heat exchanger enabling heat transfer with coolant or exhaust. Next, the hot pressurized fuel is passed through a membrane unit which separates the mixed fuel into higher-octane fractions (such as ethanol) and lower-octane fractions (such as gasoline). The higher-octane fractions leave the separator as a vapor, but are condensed in a heat exchanger and pumped with an electric pump into a storage tank. Hot lower-octane fuel remains as a liquid fuel in the separator. This fraction is cooled in another heat exchanger and returned to the main fuel tank. In addition to the above-mentioned electrical components, when vehicle speed is low, electric fans may be used for the heat exchangers.
However the inventors herein have recognized potential issues with such systems. As an example, due to the large number of electrically operated components required for fuel separation, the electrical load on the vehicle system may be significant. As a result, during conditions when the engine is operating with high electrical loads, the fuel separation system may not be operated reliably. For example, there may be inefficient high currents. This can cause fuel separation to become limited. Fuel separation may also be inefficient at lower engine loads due to poor engine efficiency at lower loads, and thus poor efficiency of generating the electricity required for the separator. Thus during those conditions, the fuel economy benefit from the availability and usage of the higher octane fuel may be overshadowed by the fuel penalty associated with fuel separation at lower engine loads. Another potential issue is that the transaxle of the vehicle may not be able to operate at a speed-load condition that is optimal for a given fuel fraction. Likewise, during changes in driver demand, there may be rapid speed-load transitions which can cause the engine to operate at a less than optimal point for the given fuel. All of these issues result in the optimal fuel economy benefit of the on-board fuel separation system not being realized.
The inventors herein have recognized that by integrating a fuel separation system into a hybrid vehicle system, various synergies can be achieved. For example, the hybrid technology can enable the engine to be scheduled to operate at different engine speed-load points depending on which fuel is selected for use. In addition, the higher voltage system architecture of the hybrid vehicle can improve the efficiency of the electric fuel separator. In one example, potential synergies are attained by a method for a hybrid vehicle including an engine, comprising: transferring engine output to a generator, and supplying electric power from the generator to an electric fuel separator without the power required to operate the electric fuel separator being stored in a battery; and separating a fuel into higher octane and lower octane fractions at the separator. In addition, separator output may be opportunistically increased during selected conditions, such as during low load conditions and during regenerative braking. In this way, fuel economy of a vehicle can be enhanced.
As an example, a hybrid vehicle system may be configured with a battery powered electric motor (or motor/generator) for propelling vehicle wheels via motor torque, as well as an engine for propelling vehicle wheels via engine torque, the engine including an on-board electric fuel separator. The fuel separator may be operated during engine running conditions to separate fuel in the fuel tank into higher and lower octane fuel fractions. The engine may then be operated with one or more of the higher and lower octane fuel fractions. In particular, the engine may be operated to generate sufficient torque to propel the vehicle and operate the motor, the motor output then used to drive the fuel separator, the electric power for the fuel separator not stored in a system battery. As such, the electric motor of the hybrid vehicle may have a higher output (e.g., 48V) than an electric motor used on conventional vehicles (e.g., 12V). By driving the fuel separator via the generator during engine running, the higher rated electrical system of the hybrid vehicle can be leveraged to apply a higher voltage and lower current to the electric fuel separator, making the fuel separation more fuel efficient. In addition, by running the separator from excess motor/generator power, without storing the excess motor/generator power in a battery and then later extracting this power from the battery, efficiency losses associated with battery charging/discharging can be reduced. Fuel separator operation may also be opportunistically increased during selected conditions, such as when the vehicle electrical demand is lower, when the engine load is at or near a minimum load, or when excess electrical energy is available at the vehicle (e.g., during regenerative braking). During such conditions, the fuel separator output (e.g., speed or pressure) may be opportunistically increased to maximize fuel separation. In one example, by increasing the fuel separator pressure when the engine load is low, the engine load may be raised. The added electrical load may enable the engine to be operated at a more efficient speed-load, and therefore with higher fuel economy. The fuel separator may be disabled during engine running conditions when the vehicle electrical demand is higher. If a sufficient amount of fuel has already been separated, further fuel separation may be disabled when the engine is not running (such as when the vehicle is propelled by motor torque only).
Furthermore during engine operation, the electric motor of the hybrid vehicle and/or a continuously variable transmission may be leveraged to operate the engine with an engine speed-load profile optimized for the fuel being used while providing the driver torque demand. As an example, when operating the engine on a higher octane fuel fraction, a speed ratio of the CVT may be selected that operates the engine at a lower engine speed and a higher engine torque (for a given power level) to leverage the greater knock resistance and higher efficiency of the higher-octane fuel. As another example, when operating the engine on a lower octane fuel fraction, if the engine is knock limited, the CVT speed ratio may be used to operate the engine at a higher engine speed and a lower engine torque (for the given power level) to reduce the amount of spark retard (and the associated fuel penalty) required for knock mitigation. Furthermore, if the engine is knock limited, some battery power may be used to reduce the engine power/torque, so as to operate the engine at a same engine speed and a lower engine torque (for the given driver requested vehicle power level) to reduce the amount of spark retard (and the associated fuel penalty) required for knock mitigation.
In this way, fuel economy in a hybrid vehicle system can be improved. One of the technical effects of integrating on-board fuel separation technology in a hybrid vehicle is that an electrically powered fuel separator may be operated more reliably even as electrical loads vary. By driving the electrical fuel separator from the generator without going through the battery, fuel separation may be achieved while efficiency losses associated with charging and discharging a system battery are reduced. By leveraging the higher voltage of the hybrid vehicle for on-board fuel separation, lower currents and higher efficiencies can be achieved by the fuel separator. By improving the reliability of fuel separation, fuel usage and thereby fuel economy is improved. Furthermore, by scheduling an engine speed-load for a given fuel via adjustments to the hybrid motor and/or a continuously variable transmission, knock limitations are reduced, further improving fuel economy. The technical effect of using the higher voltage system of a hybrid electric vehicle for operating an electric fuel separator of an engine is that fuel separation can be completed when required with reduced parasitic losses, because the higher voltage system operates at lower current, which result in lower losses because electrical power losses are proportional to the square of current (Ploss=i2 R). In addition, usage of a selected fuel fraction can be extended despite changes in driver or wheel torque demand.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.